Optical systems typically include many components, which interact to generate, transmit, modify, and detect light. Light may be generated by light sources, transmitted by optical relay structures, and detected by detectors. Light may be modified by optical filters positioned in an optical path in one or both of the light source and detector ends of the instrument.
Optical filters modify the intensity, spectrum, polarization, and other properties of light. "Intensity filters" modify the intensity of light, where intensity is the amount of light per unit area per unit time. Intensity filters may absorb light, dissipating the absorbed energy as heat, or they may reflect or scatter light. "Spectral filters" modify the spectrum of light, where spectrum is the wavelength composition of light. Spectral filters may selectively transmit light of preselected wavelengths and selectively absorb, reflect, or scatter light of other wavelengths. A spectral filter may convert light of many colors into light of one or only a few colors. "Polarization filters" modify the polarization of light, where polarization is the direction of the electric field associated with light.
Different applications or conditions may require different optical filters. For this reason, filter holders have been developed that allow one of a plurality of optical filters to be selected and placed in an optical path. Examples include filter wheels and filter slides. Unfortunately, these filter holders have a number of shortcomings. In particular, the number of optical filters required even for a single application often exceeds the filter-holding capacity of a given filter holder. Therefore, it sometimes is necessary to replace the optical filters within a given filter holder.
Replacing optical filters may be difficult and time-consuming. If individual optical filters are affixed permanently to the filter holder, the entire filter holder may need to be replaced. If individual optical filters are affixed to removable filter cartridges within the filter holder, the filter holder still must be opened, individual filter cartridges removed and replaced, and the filter holder closed again. In known filter holders, filter cartridges must be replaced with the filter holder attached to an associated instrument. Working space may be minimal, and filter cartridges and other components may be dropped into the instrument, where they may cause damage and be difficult to retrieve.
Replacing optical filters within filter cartridges also may be difficult and time-consuming. Many or most optical filters are permanently affixed to any associated filter cartridge, and may not be replaced at all. Other optical filters may be removably affixed to an associated filter cartridge, but replaceable only with a limited selection or number of filters. Filter cartridges with removable optical filters may employ a retaining ring that fits into a groove on the inside of the filter cartridge to hold the optical filter. The groove establishes a predetermined position for the retaining ring, and may limit the thickness or number of replacement filters. Optical filters that are thicker than the provided space will not fit, and optical filters that are thinner than the provided space may require spacers. Filter cartridges with removable optical filters also may employ a threaded retention member that can be screwed into the filter cartridge until a point where it holds the optical filter. This approach may require extra tools and be time-consuming. This approach also may force the retaining ring into a plane that is slightly skewed relative to the filter, misaligning the optical filter.